Specific Immunity

Specific (adaptive) immunity is not present at birth; it is acquired. As a person's immune system encounters antigens, it learns the best way to attack each antigen and begins to develop a memory for that antigen. Specific immunity is so named because it tailors its attack to a specific antigen previously encountered. The hallmarks of specific immunity are its ability to learn, adapt, and remember. Specific immunity takes time to develop after initial exposure to a new antigen. However, because a memory is formed, subsequent responses to a previously encountered antigen are more effective and more rapid than those generated by nonspecific immunity.

Lymphocytes are the most important type of white blood cell involved in specific immunity. Dendritic cells, antibodies, cytokines, and the complement system (which enhances the effectiveness of antibodies) are also involved.

Lymphocytes

Lymphocytes enable the body to remember antigens and to distinguish self from nonself (foreign). Lymphocytes circulate in the bloodstream and lymphatic system and move into tissues as needed.

The immune system can remember every antigen encountered because lymphocytes live a long time--for years or even decades. When lymphocytes encounter an antigen for the second time, they respond quickly, vigorously, and specifically to that particular antigen. This specific immune response is the reason that people do not contract chickenpox or measles more than once and that vaccination can prevent certain disorders.

Lymphocytes include B lymphocytes, T lymphocytes, and natural killer cells (which are involved in nonspecific immunity).

B Lymphocytes: B lymphocytes (B cells) are formed in the bone marrow. B lymphocytes have particular sites (receptors) on their surface where specific antigens can attach. When a B lymphocyte encounters an antigen, the antigen attaches to the receptor, stimulating the B lymphocyte to change into a plasma cell. Plasma cells produce antibodies. These antibodies are specific to the antigen that stimulated their production.

T Lymphocytes: T lymphocytes (T cells) are produced in the thymus gland. There, they learn how to distinguish self from nonself. Only the T lymphocytes that tolerate the self-identification molecules are allowed to mature and leave the thymus. Without this training process, T lymphocytes could attack the body's cells and tissues.

Mature T lymphocytes are formed and stored in secondary lymphoid organs (such as the spleen), bone marrow, and lymph nodes. They circulate in the bloodstream and the lymphatic system, where they search for particular foreign or abnormal cells, such as particular bacteria or cells infected by particular viruses. T lymphocytes can attack particular foreign or abnormal cells.

There are different types of T lymphocytes:

Killer (cytotoxic) T cells attach to foreign or abnormal cells (because they recognize the antigens on these cells). Killer T cells kill foreign or abnormal cells by making holes in the cell membrane and injecting enzymes into the cells.
Helper T cells help B lymphocytes recognize and produce antibodies against foreign antigens. Helper T cells also help killer T cells kill foreign or abnormal cells.
Suppressor T cells produce substances that help end the immune response.

Sometimes T lymphocytes--for reasons that are not completely understood--develop without or lose the ability to distinguish self from nonself. The result is an autoimmune disorder, in which the body attacks its own tissues (see Section 16, Chapter 186).

Dendritic Cells

Dendritic cells develop from monocytes and reside mainly in tissues. Newly developed dendritic cells ingest and break antigens into fragments so that other immune cells can recognize them--an activity called antigen processing. A dendritic cell matures after it is stimulated by cytokines at a site of infection or inflammation. Then, it moves from tissues to the lymph nodes where it shows (presents) the antigen fragments to T lymphocytes, which generate a specific immune response.

Antibodies

When a B lymphocyte encounters an antigen, it is stimulated to mature into a plasma cell, which then produces antibodies (also called immunoglobulins, or Ig). Antibodies protect the body by helping other immune cells ingest antigens, by inactivating toxic substances produced by bacteria, and by attacking bacteria and viruses directly. Antibodies also activate the complement system. Antibodies are essential for fighting off certain types of bacterial infections.

See the figure Basic Y Structure of Antibodies.

Each antibody molecule has two parts. One part varies; it is specialized to attach to a specific antigen. The other part is one of five structures, which determines the antibody's class--IgG, IgM, IgD, IgE, or IgA. This part is the same within each class.

IgM: This class of antibody is produced when a particular antigen is encountered for the first time. The response triggered by the first encounter with an antigen is called the primary antibody response. Normally, IgM is present in the bloodstream but not in the tissues.

IgG: The most prevalent class of antibody, IgG is produced when a particular antigen is encountered again. This response is called the secondary antibody response. It is faster and results in more antibodies than the primary antibody response. IgG is present in the bloodstream and tissues. It is the only class of antibody that crosses the placenta from mother to fetus. The mother's IgG protects the fetus and infant until the infant's immune system can produce its own antibodies.

IgA: These antibodies help defend against the invasion of microorganisms through body surfaces lined with a mucous membrane, including those of the nose, eyes, lungs, and digestive tract. IgA is present in the bloodstream, in secretions produced by mucous membranes, and in breast milk.

IgE: These antibodies trigger immediate allergic reactions (see Section 16, Chapter 185). IgE binds to basophils (a type of white blood cell) in the bloodstream and mast cells in tissues. When basophils or mast cells with IgE bound to them encounter allergens (antigens that cause allergic reactions), they release substances that cause inflammation and damage surrounding tissues. Thus, IgE is the only class of antibody that often seems to do more harm than good. However, IgE may help defend against certain parasitic infections that are common in some developing countries.

IgD: Small amounts of these antibodies are present in the bloodstream. The function of IgD is not well understood.

See the sidebar Strategies for Attack.